There are many applications in research and industry where a short pulse of an amplified optical beam is required at a particular frequency or wavelength. One way in which to generate a short pulse at a desired wavelength is to utilize a lasing medium which is adapted to lase over a relatively broad wavelength band, to mode lock the laser and to then tune the laser to generate its output at the desired wavelength. For example, Ti sapphire lasers are adapted to lase over a spectrum or band from 650 to 1050 nm, a wavelength and where few other lasers are available.
However, some difficulties have been experienced in tuning lasers in general, and Ti sapphire lasers in particular, to a desired output wavelength. This has been true when operating in continuous wave mode, and is even more true when operating in pulse mode, particularly narrow pulse mode.
One technique which has been utilized in the past to obtain short tunable pulses from a laser is to apply an optical seeding signal to the laser at the desired frequency and of desired pulse width. A system for operating a Ti sapphire laser in this manner is shown in an article entitled "64-dB amplification of 19-psec laser diode pulses in a Ti-sapphire laser" by Santanu Basu, Paul May and Jean Marc Halbout, Optics Letters, Vol. 14, No. 22, Nov. 15, 1989, Page 1272-1274. The system disclosed in this article utilizes a diode laser operating at the desired frequency to generate the required seeding signal.
While the system shown in the Basu et al article is useful, it has a number of limitations which restrict its utility as a commercial product. First, it may be difficult to find a diode laser operating in the desired wavelength band and second, the tuning range utilizing such a diode laser may be limited. In addition, the diode laser needs to be mode matched and synchronized with the laser it is seeding in order for the desired laser frequency control to be achieved. Such synchronization is not easily achievable, making this system difficult to use. Finally, energy available in a short pulse from a diode laser for generating the seeding signal is usually so small as to make the seeding function difficult.
Other techniques for generating short pulses from tunable lasers have involved similar problems of achieving tunability over a broad band, achieving synchronization, etc., and some schemes have also involved the use of relatively large and expensive Argon ion lasers (see, for example, "Amplification of Femtosecond Pulses in Ti:Al/sub 2/0/sub 3/ Using an Injection Seeded Laser", Dept. of Electr. Eng., MIT, Cambridge, MA 15, Dec. 1989, LaGasse, M.J., et al).
A need, therefore, exists for an improved method and apparatus for tuning Ti sapphire and other lasers adapted to generate outputs over a broad wavelength band, which method and apparatus affords relatively easy tunability over the entire wavelength band of the laser, which provide inherent stability and synchronization between the laser and any seeding signal and which utilize only relatively small and relatively low power solid state components.